Has to be a letter from your HR department confirming your employment, wage, employment dates, etc. If you work for a big corporation you should be able to contact your main HR department or go to the internal company HR site and print this information or request it. If you don't have a HR department have your boss print a letter with a real company letterhead and professionally written stating your time/date of your employment, hours you work per week and your salary.
Don't forget you'll need your Notice of Assessment as well from the CRA.
Good luck.
*edit*-
If your application got sent back unprocessed it won't affect it, fix the mistakes and resend it. Good luck.